Evaluation of a quality of sleep, which is one of comfort levels of a subject, is important in diagnosis and medical treatment of various diseases.
Including a period of being awake, sleep of a human includes six stages, that is, an arousal period, a REM period (Rapid Eye Movement: sleep period during which eye movement is found), a NREM (non-REM) period, first stage (inclined sleep initial stage), a NREM period, second stage (inclined sleep stage), a NREM period, third stage (moderate sleep stage), and a NREM period, fourth stage (deep sleep stage).
In a normal sleep, when entering a sleep state from an arousal period, transition of a sleep state is repeated three cycles in one night, each cycle being 90 minutes typically or 60 to 120 minutes in general, called a sleep cycle (ultradian rhythm), and each cycle includes a part of or all of the above-described stages in the REM period and the NREM period, in which the depth of sleep cyclically (periodically) changes in each cycle and also changes with a tendency of gradually changing from a deep sleep state in the sleep initial stage to a light sleep in the whole sleep in one night.
Therefore, the level of comfort including the quality of sleep is evaluated on the basis of whether the cycle of sleep repeated in this ultradian rhythm is clearly found or in each cycle, whether cyclic sleep stage transition is clearly found in each cycle or whether the depth of sleep gradually changes from the sleep initial stage to a light end stage in the whole sleep in one night.
In a sleep with a non-favorable quality, the ultradian rhythm is not clear in the transition of the sleep state, and there can be a case in which a deep sleep stage is not found in the sleep initial stage but to the contrary, the deep sleep stage comes in the end stage, for example.
There are various diseases that cause obstruction in a good-quality sleep, and in OSAS (Obstructive sleep apnea syndrome), for example, a tongue part of a patient during sleep lowers by a gravitational force and physically blocks an airway, which obstructs respiration and causes arousal and prevents entry to a deep sleep stage.
Also, CSR (Cheyne-Stokes Respiration) considered to be found in approximately 40% of CHF (congestive heart failure) patients also causes a drop in the comfort level including the quality of sleep.
The CSR is respiration in which after a tidal volume gradually increases from small respiration, the tidal volume gradually decreases, and respiratory arrest (apnea for approximately 10 to 20 seconds) occurs and then, the similar cycle is repeated.
A factor to cause occurrence of CSR in the CHF patients is understood as follows.
The respiratory center of the brain executes respiratory control by detecting a CO2 partial pressure in blood in a normal time. The CHF patient has high brain sensitivity to the CO2 partial pressure while being awake and is in a hyperventilation state.
However, during sleep, this sensitivity is somewhat recovered and lowered, and thus, unless the CO2 partial pressure in blood rises higher than in arousal (that is, apnea), respiration is not started, and the above-described CSR occurs.
The Cheyne-Stokes Respiration symptom is often observed in the CHF and is accompanied by sleep disorder caused by a nocturnal hypoxia state and arousal. The nocturnal hypoxia state and arousal cause an increase in a pulmonary artery pressure and sympathetic nerve activities, lowers exercise tolerance and deteriorates prognosis.
As described above, since the comfort level including the quality of sleep is lowered due to various diseases, it is necessary to evaluate the comfort level including the quality of sleep of a subject and to utilize the result for diagnosis and treatment.
First, a prior-art method of evaluating the comfort level including the quality of sleep will be described.
In the past, in order to evaluate the comfort level including the quality of sleep, the following sleep test using a device called PSG (Polysomnography) (hereinafter, this sleep test is called “PSG” or “PSG test”) has been conducted in general. The PSG is a test in which a medical staff quantitatively evaluates the depth of sleep (sleep stage), fragmentation of sleep, presence of arousal reactions and the like by measuring respiratory flows, snoring sound, oxygen saturation in blood (SpO2), brain waves, electromyograms, eye movement and the like over a sleep period of a subject.
The medical staff identifies the sleep cyclic period from a change in the brain waveform, for example, using the measurement result of the PSG and makes evaluations by means of a method such as discrimination between the REM period and the NREM period from the presence of eye movement and surface electromyography. These PSGs are disclosed in the following Patent Document 1 and Patent Document 2, for example.
Also, though it is different from the PSG, Patent Document 3 describes a method, as indicated in the paragraph 0023, in which respiratory data and motion data such as roll-over in each sleep stage of a subject are accumulated in advance by using the PSG and the current sleep stage is identified only from the respiratory data and the motion data in a test not using the PSG. Execution of the PSG is needed for creating initial data for identification, and identification accuracy is an important issue in the work of identification of the sleep stage from the measurement data . . .
Next, a prior-art technology relating to observation and detection of the Cheyne-Stokes respiration will be described.
In the detection of the Cheyne-Stokes respiration, the above-described PSG has been used in general. That is, the brain waves, eye movement, respiratory flows, ventilation motion by thoracoabdominal movement, arterial oxygen saturation, electrocardiogram (including heart rate) and the like are measured during the sleep period at night using PSG, and if gradual increase and gradual decrease of respiratory flows and respiratory efforts are found to occur repeatedly during the NREM sleep 1 to 2 (light sleep) from the measurement result report, the medical staff makes a diagnosis that occurrence of the Cheyne-Stokes respiration is suspected or the like.
With the purpose of simplified and reliable discovery of such Cheyne-Stokes respiration, Teijin Limited has proposed a biological information monitoring device with which a medical staff can observe the Cheyne-Stokes respiration symptom from an analysis result of the measurement result of an autonomic nerve alternation state on the basis of heart-rate alternation analysis and the measurement results of respiratory flows and respiratory efforts (ventilation motion), and the configuration is disclosed in Patent Document 4.
However, these prior-art technical configurations for detecting the Cheyne-Stokes respiration are all used such that the medical staff observes physiological data and detects the Cheyne-Stokes respiration. That is, though the Cheyne-Stokes respiration is recognized as an important risk factor in chronic heart failure, a configuration of automatically detecting occurrence of the Cheyne-Stokes respiration has not been proposed up to now.